Gerhard Wagenblast

High‐Efficiency Blue and White Organic Light‐Emitting Devices Incorporating a Blue Iridium Carbene Complex

High-effi ciency white organic light-emitting devices (OLEDs) have great potential for energy saving solid-state lighting and eco-friendly fl at-display panels. [ 1 ] In addition, white OLEDs are expected to open new designs in lighting technology, such as transparent lighting panels or luminescent wallpapers because of being able to form paper-like thin fi lms. Phosphorescent OLED technology is an imperative methodology to realize higheffi ciency white OLEDs because phosphors, such as fac -tris(2phenylpyridine)iridium( III ) [Ir(ppy) 3 ] and iridium( III )bis(4,6(difl uorophenyl)pyridinatoN , C 2 ′ )picolinate (FIrpic) enable an internal effi ciency as high as 100% converting both singlet and triplet excitons into photons. [ 2 ] There are two effective approaches to obtain white OLEDs by using phosphors. One is to combine a blue fl uorophore and phosphors for the other colors, a so-called hybrid white OLED. [ 3 ] A key requirement is the use of a blue fl uorophore with higher triplet energy ( E T1 ) than that of the other phosphors. The blue fl uorophore also needs to have a high photoluminescent quantum yield ( η PL ). Schwartz and coworkers reported hybrid white OLEDs with a power effi ciency at 1000 cd m − 2 ( η p,1000 ) of 22 lm W − 1 (external quantum effi ciency (EQE) of 10.4%) by using N , N ′ -di-1-naphthalenylN , N ′ -diphenyl-[1,1 ′ :4 ′ ,1 ′ ′ :4 ′ ′ ,1 ′ ′ ′ -quaterphenyl]-4,4 ′ ′ ′ -diamine (4P-NPD) as a blue fl uorophore, and Ir(ppy) 3 and iridium( III ) bis(2-methyldibenzo-[ f , h ]quinoxaline)(acetylacetonate) [Ir(MDQ) 2 ( acac)] as green and red phosphors, respectively. [ 4 ]

Enlarging the π System of Phosphorescent (C^C*) Cyclometalated Platinum(II) NHC Complexes

Cyclometalated (C^C*) platinum(II) N-heterocyclic carbene (NHC) complexes are emerging as a new class of phosphorescent emitters for the application in organic light-emitting devices (OLEDs). We present the synthesis of six new complexes of this class to investigate the influence of extended π systems. Therefore, six different NHC ligands with a varying number of additional phenyl substituents were used in combination with the monoanionic acetylacetonate (acac) ligand to obtain complexes of the general formula [(NHC)Pt(II)(acac)]. The complexes were fully characterized by standard techniques and advanced spectroscopic methods ((195)Pt NMR). For all complexes the solid-state structure determination revealed a square-planar coordination of the platinum atom. Absorption and emission spectra were measured in thin amorphous poly(methyl methacrylate) films at room temperature. Four compounds emit in the blue-green region of the visible spectrum with quantum yields of up to 81%.

Heteroleptic platinum(II) NHC complexes with a C^C* cyclometalated ligand – synthesis, structure and photophysics

Platinum(II) complexes [(NHC)Pt(L)] with various β-diketonate based auxiliary ligands (L: 3-meacac = 3-methylacetylacetonato, dpm = dipivaloylmethanato, dbm = dibenzoylmethanato, mesacac = dimesitoylmethanato, duratron = bis(2,3,5,6-tetramethylbenzoyl)methanato) and a C^C* cyclometalated N-heterocyclic carbene ligand (NHC: dpbic = 1,3-diphenylbenzo[d]imidazol-2-ylidene, dpnac = 1,3-diphenylnaphtho[2,3-d]imidazol-2-ylidene or bnbic = 1-phenyl-3-benzylbenzo[d]imidazol-2-ylidene) were found to show different aggregation and photophysical properties depending on the auxiliary ligand. Eight complexes were prepared from a silver(I)–NHC intermediate by transmetalation, cyclometalation and subsequent treatment with potassium-tert-butanolate and β-diketone. They were fully characterized by standard techniques including 195Pt NMR. Five complexes were additionally characterized by 2D NMR spectroscopy (COSY, HSQC, HMBC and NOESY). Solid-state structures of five complexes could be obtained and show the tendency of the square-planar compounds to form pairs with different Pt–Pt distances depending on the bulkiness of the substituents at the auxiliary ligand. The result of the photophysical measurements in amorphous PMMA films reveals quantum yields of up to 85% with an emission maximum in the blue region and comparatively short decay lifetimes (3.6 μs). Density functional theory (DFT/TD-DFT) calculations were performed to elucidate the emission process and revealed a predominant 3ILCT/3MLCT character. Organic light-emitting devices (OLEDs) comprising one of the complexes achieved 12.6% EQE, 11.9 lm W−1 luminous efficacy and 25.2 cd A−1 current efficiency with a blue emission maximum at 300 cd m−2. The influence of an additional hole-transporter in the emissive layer was investigated and found to improve the device lifetime by a factor of seven.

Phosphorescent Platinum(II) Complexes with Mesoionic 1H-1,2,3-Triazolylidene Ligands

The synthesis and characterization of eight unprecedented phosphorescent C^C* cyclometalated mesoionic aryl-1,2,3-triazolylidene platinum(II) complexes with different β-diketonate ligands are reported. All compounds proved to be strongly emissive at room temperature in poly(methyl methacrylate) films with an emitter concentration of 2 wt %. The observed photoluminescence properties were strongly dependent on the substitution on the aryl system and the β-diketonate ligand. Compared to acetylacetonate, the β-diketonates with aromatic substituents (mesityl and duryl) were found to significantly enhance the quantum yield while simultaneously reducing the emission lifetimes. Characterization was carried out by standard techniques, as well as solid-state structure determination, which confirmed the binding mode of the carbene ligand. DFT calculations, carried out to predict the emission wavelength with maximum intensity, were in excellent agreement with the (later) obtained experimental data.

Phosphorescent Platinum(II) Complexes with C^C* Cyclometalated NHC Dibenzofuranyl Ligands: Impact of Different Binding Modes on the Decay Time of the Excited State

Two C^C* cyclometalated platinum(II) N-heterocyclic carbene (NHC) complexes with the general formula [(C^C*)Pt(O^O)] (C^C*=1-dibenzofuranyl-3-methylbenzimidazolylidene; O^O=dimesitoylmethane) have been synthesized and extensively characterized, including solid-state structure determination, (195) Pt NMR spectroscopy, and 2D NMR (COSY, HSQC, HMBC, NOESY) spectroscopy to elucidate the impact of their structural differences. The two regioisomers differ in the way the dibenzofuranyl (DBF) moiety of the NHC ligand is bound to the metal center, which induces significant changes in their physicochemical properties, especially on the decay time of the excited state. Quantum yields of over 80 % and blue emission colors were measured.

Binuclear C^C* Cyclometalated Platinum(II) NHC Complexes with Bridging Amidinate Ligands

Binuclear C^C* cyclometalated NHC platinum(II) compounds with bridging amidinate ligands were synthesized to evaluate their photophysical properties. Their three-dimensional structures were determined by a combination of 2D NMR experiments, mass spectrometry, DFT calculations, and solid-state structure analysis. The bridging amidinate ligands enforce short distances between the platinum centers of the two cyclometalated structures, which gives rise to extraordinary photophysical properties.

Changing the Emission Properties of Phosphorescent C^C*-Cyclometalated Thiazol-2-ylidene Platinum(II) Complexes by Variation of the β-Diketonate Ligands.

Cyclometalated thiazol-2-ylidene platinum(II) complexes based on the N-phenyl-4,5-dimethyl-1,3-thiazol-2-ylidene N-heterocyclic carbene (NHC) ligand and seven different β-diketonate ligands have been synthesised and investigated for their structural and photophysical properties. The complexes were synthesised in a one-pot procedure starting with the in situ formation of the corresponding silver(I) carbene and transmetalation to platinum, followed by the reaction with the respective β-diketonate under basic conditions. All the compounds were fully characterised by standard techniques, including 195 Pt NMR spectroscopy. Three solid-state structures revealed quite different aggregation behaviour depending on the β-diketonate architecture. The reported complexes showed strong phosphorescence at room temperature in amorphous poly(methyl methacrylate) films. The emission wavelengths (ca. 510 nm) were found to be independent of the β-diketonate ligand, but the electronically diverse β-diketonates strongly influence the observed quantum yields (QY) and decay lifetimes. The results of theoretical studies employing density functional theory (DFT) methods support the conclusion of a metal-to-ligand charge transfer (3 MLCT) as the main emission process, in accordance with the reported photophysical properties. Standard organic light-emitting diodes (OLEDs) prepared with unoptimised matrix materials using one of the complexes showed values of 12.3 % external quantum yield, 24.0 lm W-1 luminous efficacy and 37.8 cd A-1 current efficiency at 300 cd m-2 .

Host Materials for Blue Phosphorescent OLEDs

PCTrz, a new bipolar host material containing a phenoxy-carbazole separated from a biscarbazolyl-triazine by a non-conjugated ether bond is presented. Computational calculations demonstrated the separation of PCTrz into an oxidation and a reduction site. A phosphorescent OLED with PCTrz as host and FIrpic as blue emitter yielded high current efficiencies of up to 16.2 cd/A. Additionally two electron transporting host materials DBFTaz and DBFTazC, both containing 1,2,4-triazole moieies, were synthesized and characterized. The triazole moiety in DBFTaz was formed by a classical ring closure reaction between a N-benzoylbenzimidate and a hydrazine. For DBFTazC we used another synthetical pathway which involves subsequent coupling of a carbazole and a triazole moiety to a dibenzofuran core. Both triazoles posses high triplet energies of 2.95 eV for DBFTaz and 2.97 eV for DBFTazC, which make the compounds interesting as matrix materials for blue phosphorescent OLEDs.

Controlling the radiative rate of electrophosphorescent organometallic complexes by engineering the singlet-triplet splitting

We address the role of singlet-triplet splitting in controlling the radiative rate for deep-blue electrophosphorescent metal complexes. An enhanced radiative rate correlates with a small splitting, highlighting the road map to efficient electrophosphorescence.

Deep Blue Phosphorescent OLEDs with Improved Device Stability

Cyclometallated iridium N-heterocyclic carbene (NHC)-complexes have become known as efficient deep blue triplet emitters in OLEDs. Herein we discuss new materials and device setups for carbene-based deep blue OLEDs with improved stability and lifetime.